Micromachined ultrasonic transducer is an electronic device that comprises vibrating membranes capable of generating high pressure waves with an applied AC voltage signal. The generated waves travel through a medium and reflect back to the transducer when the waves meet an object. The transmitted and returned waves are electronically processed to detect information about the object including its distance, shape and other physical properties. The vibration of the membrane is enabled by either capacitive or piezoelectric transduction. Capacitive micromachined ultrasonic transducers are known as CMUT, whereas piezoelectric micromachined ultrasonic transducers are known as PMUT.
Wafer bonding is a conventional method for integrating micromachined ultrasonic transducers with complementary metal oxide semiconductor, CMOS substrates. US20160009544A1 discloses a method of integrating micromachined ultrasonic transducers with complementary metal oxide semiconductor, CMOS substrates. The micromachined ultrasonic transducers can be CMUT or PMUT. This method involves the wafer bonding technique to bind the PMUT to the substrate, as well as to connect one substrate to another. WO2016040333 describes a microelectromechanical system, MEMS device having a PMUT which is formed by an ultrasound transducer, MUT structure and a piezoelectric material. A first metal conductive layer is disposed on the piezoelectric material and a plurality of metal electrodes is configured to form electrical connections between the first metal conductive layer, the piezoelectric material, and a CMOS structure. The PMUT structure and the CMOS structure are vertically stacked, whereby the MUT structure is bonded to the CMOS structure at the standoff via wafer bonding techniques such as eutectic bonding and compression bonding.
This disclosure focuses on the micromachined ultrasonic transducer that utilizes the piezoelectrically actuated membrane. PMUT operates based on the flexural motion of a thin membrane that is coupled with a thin piezoelectric film. Various advantages offered by PMUT include increase bandwidth, provide flexible geometries, minimize voltage requirements, enable mixing of different resonant frequencies and support miniaturization of high frequency electronic devices. Typical PMUT cells are fabricated on a separate substrate and wire bond to the CMOS substrate or bond to the CMOS substrate using wafer bonding techniques. However, these bonding methods cause low fill-factor and high amount of electrical parasitic.
To enhance mechanical integrity of two connected PMUT chips, a bonding ring which is typically 10-15 um is applied around the effective PMUT area, in between the CMOS substrate and the piezoelectric. In addition, to increase electrical integrity, an additional metal line with a width of approximately 5 um is applied for connecting the piezoelectric of two adjacently positioned PMUT chips. When multiple PMUT cells are populated within a specific area, the bond ring and electrical ring occupy significant area and thus causes low fill-factor of the integrated PMUT-CMOS device.
The invention disclosed herein shall provide a solution to the abovementioned limitations and drawbacks of current integrated or connected PMUT-CMOS devices. By this invention, the PMUT cells can be placed closely to each other and improvement of the fill-factor can be achieved.